Stabilized scintillation type radiation detector



Dec. 2l, 1965 s, A. scHERBATsKOY 3,225,195

STABILIZED SCINTILLATION TYPE RADIATION DETECTOR 2 Sheets-Sheet 1 Original Filed Aug.

20T-.030mm wl mph@ DeC- 21, 1965 s. A. SCHERBATSKOY 3,225,195

STABILIZED SCINTILLATION TYPE RADIATION DETECTOR 2 Sheets-Sheet 2 Original Filed Aug. 17, 1959 Fig. Ib

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United States Patent O 3,225,195 STABILIZED SCINTILLATION TYPE RADIATION DETECTOR Serge A. `Scherhatslroy, 804 Wright Bldg., Tulsa 3, Okla. @riginal application Aug. 17, 1959, Ser. No. 834,182, now Patent No. 3,089,955, dated May 14, 1963. Divided and this application Mar. 25, 1963, Ser. No. 267,430 2 Claims. (Cl. 250-71.5)

This invention relates to the detection and measurement of nuclear radiation, and more particularly to an improved form of stabilized scintillation counter.

The present specification is a division of my copending application Serial No. 834,182, tiled August 17, 1959, now U.S. Patent No. 3,089,955.y

The prime object of the present invention is to provide an improved scintillation detector of the type wherein sensitivity variations in the photomultiplier tube are compensated by means including a pilot source of standard light flashes, such standard flashes in the present invention being provided by a scintillating phosphor having a radioactive substance at least partially dispersed therein.

Other objects and advantages of the invention will appear from the following detailed description and drawings of a typical embodiment thereof.

FIG. 1 illustrates an embodiment of my invention `in which provision has been made for varying the selectivity of an `amplitude-selective network in response to the variation in sensitivity of a photomultiplier.

FIGS. 1a and 1b illustrate certain pulse shapes produced by part of the apparatus of FIG. 1.

FIG. 2 illustrates diagrammatically a succession of current impulses obtained across the output terminals of the photomultiplier.

FIG. 3 shows schematically an electric circuit for transmitting impulses within a predetermined range of magnitude.

FIG. 4 shows diagrammatically the output of a pulseshaping network.

In the present invention, variations in photomultiplier sensitivity are compensated for by continuously comparing the intensity of the light flashes generated in a scintillation phosphor to the intensity of standard reference light pulses. The source of these standard reference light pulses, in the present invention, is a small scintillating phosphor having dispersed therein, as by inclusion in the rnelt, the radioactive material that emits monochromatic charged particles. These particles interact with the material of the phosphor and thereby produce a succession of light pulses having uniform intensity, directly related to the energy of the charged particles emitted by the radioactive material.

More specifically, the source of reference light pulses may consist of a small crystal of thallium-activated sodium iodide which contains dispersed therein a small amount of plutonium 239. The reference light pulses are produced within the sodium iodide crystal as the result of absorption of the 5.1 m.e.v. alpha rays emitted by the plutonium.

In such an arrangement a photomultiplier is placed in optically coupled relation to the small crystal containing the radioactive element which serves as a source of reference pulses. In addition to said small crystal, a relatively much larger scintillating crystal is also optically coupled to the photomultiplier, the purpose of the large crystal -being to detect incident rediation and to generate light flashes of corresponding magnitudes, said flashes acting on the photomultiplier and causing it to produce current impulses.

Instead of plutonium 239, the small crystal may contain some other radioactive element such as uranium 238,

3,225,195 Patented Dec. 2li, 1965 plonium 210, ionium 230 (isolated from its daughter products), radium D 210 in equilibrium with its daughter products, or any other radioactive substance emitting highly ionizing particles.

In the arrangement as described above, the photomultiplier is exposed simultaneously to the reference iiashes originated by the 5.1 m.e.v. alpha rays from plutonium and to the ashes derived from the incident radiation which it is desired to measure,I and it hence produces two sets of current pulses corresponding to the two types of ashes.

It is of course desirable that the photomultiplier output pulses representing the standard reference llashes be easily distinguishable from the output pulses representing the signal flashes in the large crystal. To `this end, it is desirable to provide means of insuring that the output pulses representing standard light flashes be substantially larger in magnitude than any of the signal pulses resulting from nuclear radiation being studied. By this technique, it is possible to separate the reference pulses from the signal pulses by simple circuitry such as a threshold network.

Referring now more particularly to FIG. 1, the arrangement shown therein comprises two thallium-activated sodium iodide crystals placed in cooperative relationship with the photomultiplier 12. One: of these crystals of relatively large size contained within the housing 11, and designated by the numeral 20, .is the main crystal, i.e., the sensing element of the instrument. It is exposed to the incident signal radiation of which detection and analysis is desired. As a result of interactions with such radiation, light flashes are produced within the crystal 20, and these ashes in turn act upon the photomultiplier 12, whereby the photomultiplier generates current pulses having magnitudes representing the energy of the radiation particles and or quanta absorbed in the sodium iodide crystal Ztl.

The other crystal 70 cooperating with the photomultiplier 12 is of relatively small size and contains a radioelement in the melt, as a result of which it is made to interact continually with the rays emitted by this radioelement; for example, the 5.1 m.e.v, alpha particles emitted by plutonium. As a result of this interaction the auxiliary crystal continually emits pulses oi light of substantially constant magnitude, the magnitude of each of said pulses having deiinite relation to the energy of the particles emitted. The auxiliary crystal is used in this arrangement as a source of reference light dashes and its purpose is to stabilize the performance of the measuring system. On the other hand, the main crystal is used as a sensing element and its purpose is to detect the incident radiation to be measured and to produce flashes indicating the character of such radiation.

Crystal 7th is provided with a reflecting surface 70a on all sides except the one exposed to the photomultiplier and also is provided with a light shield 7Gb. The purpose of the light shield is to prevent any light from the pilot crystal from going backwards and getting reflected by reiiector 11. This light shield is made sufficiently thick so as also to prevent any undesired nuclear radiation from the crystal 70 from reaching the crystal 20.

Photomultiplier 12 is continually exposed to light flashes derived from the auxiliary crystal 70 and also those derived from the main crystal 2li. The light iiashes derived from the auxiliary crystal represent alpha particles having energy 5 .1 m.e.v., and those derived from the main crystal represent the energies of various incident particles interacting with it.

In order to insure that the output pulses from the photomultiplier 12 representing reference flashes from the auxiliary crystal 70 are substantially larger in magnitude than the pulses representing ashes derived from the main crystal Z0, I provide a light attenuator interposed between the main crystal 20 and the photomultiplier 12, whereby the flashes of light derived from the main crystal are attenuated by a suitable factor, such as one-half. The auxiliary crystal 70 is placed close to the photomultiplier 12 without any light attenuator interposed therebetween, and reference light pulses derived from the auxiliary crystal are thus not attenuated. As a result of this arrangement, the 5.1 m.e.v. standard within the auxiliary crystal produces light ashes of the same intensity that would reach the photomultiplier from the main crystal if it were irradiated by 10.2 m.e.v. alpha rays, which are above our range of interest.

As shown in FIG. l the optical attenuator interposed between the main crystal 20 and the photomultiplier 12 consists of plastic light pipe 74, a light attenuator 75 adjacent to the light pipe 74, and another light pipe 76 placed between the light attenuator 75 and the transparent wall 15 of the photomultiplier. The small auxiliary crystal 70 is imbedded in the light pipe 76 and has one of its surfaces adjacent the wall 15.

The light attenuator 75 is in form of a plate made of a suitable substance having limited transparency such as Teflon. Various light :attenuators of this and other types are manufactured by Eastman Kodak Company.

It is well known that the sensitivity of a photomultiplier circuit is subject to drift variation and other unconrollable changes, an important one being due to variations in the high voltage supply. In the illustrated embodiment of my invention I vary the selectivity of the amplitude-selective network comprising the gates 141, 142, and 143 in order to compensate for variation in the sensitivity of the photomultiplier 12. This variation in selectivity is accomplished by means of a suitable controlling mechanism which utilizes the reference light flashes of constant intensity produced by crystal 70. These light flashes produce corresponding reference current pulses at the output terminals 71 of the photomultiplier 12. Because of the .presence of the optical attenuator interposed between the large crystal and the photomultiplier 12 the reference current pulses are considerably greater in magnitude than any of the current pulses at the photomultiplier output due to light flashes originating at the large crystal 20.

The magnitude of the reference current pulses serves as an index of the sensitivity of the photomultiplier. If the sensitivity of the photomultiplier 12 decreases, the pulses generated by the reference light flashes decrease in magnitude; conversely, if the sensitivity of the photomultiplier 12 increases, the pulses generated by the reference light flashes increase in magnitude. As will be fully explained hereafter, I provide controlling mechanism for varying the selectivity of the amplitude-selective network, which is responsive to increase or decrease in magnitude of the reference pulses. Thus, if the sensitivity of the photomultiplier decreases, the controlling mechanism shifts the amplitude-selective network so as to accept amplitudes of correspondingly lower values, and if the sensitivity of the photomultiplier increases, the controlling mechanism shifts the amplitude-selective networks so as to accept amplitudes of correspondingly higher values.

The photomultiplier 12 may be of a standard construction within a cylindrical enclosure 14 provided with a transparent wall 15. The photomultiplier has a photosensitive cathode 23, a plurality of dynodes 24-33, and an anode 34, each electrode being at a higher potential than the preceding one. The dynode potentials are derived from a voltage-dividing circuit consisting of a high voltage supply 35 in series with a resistor 36 and a pluarlity of resistor elements 42-52, said resistor elements being individually shunted by condensers 53-63, respectively. The voltage applied across the resistors 42-52 is approximately 1100 volts. Consequently, the

voltage applied across each of said resistors, is approximately volts. The voltage across the resistor 42 is applied between the photocathode 23 and dynode 24, the voltage across the resistor 43 is applied across the dynodes 24, 25 and the voltages across the resistors 44, 45, 46, 47, 48, 49, 50, 51 are applied across the dynodes 25-26, 26-27, 27-28, Ztl-29, 29430, 31E-31, 31-32, 32-33, respectively.

The voltage derived from the resistor 52 is applied across the electrodes 33-34 in series with the load resistor 69. The output leads 71 connect the photomultiplier output to an :amplier 72, and the output terminals of the amplier are in turn connected to the pulse-shaping network 140. For a description of a suitable pulse-shaping network see J. W. Coltman and Fitz-Hugh Marshall, Nucleonics l, 1947, p. 58.

The photocathode 23 is subjected to light impulses derived from two different sources: rst, the light pulses due to the interaction of the incident nuclear radiation 80 with the phosphor 20, and second, reference light pulses derived from the auxiliary crystal 70. Because of the optical attenuator the reference pulses are of higher intensity than the pulses resulting from the interactions of the crystal 20 with the incident nuclear radiation 80. Both these groups of pulses are amplified by the familiar secondary emission system of the multiplier tube comprising the photocathode 23 and the dynodes 24-33, each at a higher potential than the preceding one. Each photoelectron is swept to the rst dynode by a potential difference of about a hundred volts and ejects four or tive secondary electrons. These in turn are swept to the second dynode and similarly multiplied by the secondary emission amplification. After nine such stages, an avalanche of a million electrons, more or less, appears at the output of the photomultiplier tube as a result of each initial photoelectron, producing across the output terminals 71 a succession of current pulses. Some of these pulses of uniform and relatively large magnitude are due to the reference light ashes emitted by auxiliary crystal 70. Other pulses of relatively smaller and non-uniform magnitudes are caused by the interaction of incident nuclear radiation (of the beam 80) with the phosphor 20.

All of the above pulses are subsequently amplified in the amplier 72, said amplifier being connected to the pulse-shaping network which is designed to provide for each pulse a corresponding output voltage pulse that will have a somewhat longer duration (and preferably a rectangular shape) and a variable height as shown in FIG. 4, said height representing the magnitude of the impulse. The output of said pulse-shaping network therefore consists of a succession of discrete pulses, the magnitude of each pulse serving to identify the energies of in dividual photons or quanta comprised in the radiation eld 80 and the intensity of the reference pulses emitted by the auxiliary crystal 70. FIG. 2 gives a diagrammatical representation of such an output in which the abscissas represent the time of occurrence of the pulses and the ordinates represent the respective magnitudes ofthe pulses. The pulses have been designated by consecutive numerals such as 1, 2, 3, etc. These pulses have been subdivided into four energy groups which are designated by Roman numerals I, II, III, and IV.

Group I comprises pulses smaller than a predetermined value CM1 and larger than a predetermined value CM2. In FIG. 2 pulses belonging to this group are designated as 2, 11, and 13.

Group II comprises pulses smaller than a predetermined value OMZ and larger than a predetermined value OM3. In FIG. 2 the pulses belonging to this group are designated. as 9, 10, and 15.

Group III comprises pulses smaller than a predeter- In FIG. 2 the pulses belonging to this group are:

Group IV comprises pulses having all substantially a predetermined value CM1. These pulses represent the intensity of the light flashes produced by the auxiliary crystal 70. In FIG. 2 the pulses belonging to this group are designated as 3, 8, 12, and 18.

The output pulses as shown in FIG. 2 are simultaneously applied to ve gate elements designated in FIG. 1 by numerals 141, 142, 143, 144, and 145, respectively. Each gate element is characterized by two threshold values, i.e., it is arranged to transmit only those impulses the magnitude of which is below the upper threshold and above the lower threshold.

Thus the gate 141 has an upper threshold determined by the value OM1 and a lower threshold determined by the value OM2. Consequently, this gate 141 transmits only the impulses of the Group I. The gate 142 has an upper threshold determined by the value OM2 and a lower threshold determined by the value OM3. Consequently the gate 142 transmits only the impulses of the Group II. The gate 143 has an upper threshold determined by the Value OM3 and a lower threshold determined by the value OM4. Consequently the gate 143 transmits only the impulses of the Group III. The gate 144 is adapted to transmit signals having magnitude ON2 somewhat smaller than ONl, but cannot transmit signals having magnitude ONl, Consequently the upper threshold of the gate 144 is slightly above the value ON2 but below the value ONI, and the lower threshold is slightly below the value ON2. The gate 145 is adapted to transmit signals having magnitude ON3. Consequently the lower threshold of the gate 145 is slightly below the value ON3 but above the value ONl, and the upper threshold is above Ithe value ONS.

The gates 141, 142, 143, 144, and 145 are provided with control terminals 151, 152, 153, 154, and 155, respectively, that receive corresponding control voltages.

The magnitude of the control voltage applied to the terminals 151 determines the value of the thresholds OMI and OM2, By increasing (or decreasing) the control voltage the values of the thresholds OM1 and OM2 are increased (or decreased). However, the difference between the values OMI and OM2 is maintained constant. Consequently the increase (or decrease) of the control voltages causes a shift of the transmitted band of magnitudes upward toward larger values (or downward toward smaller values). However, the Width of the transmitted band is maintained constant and independent of the variation in the control voltage.

Similarly, the magnitude of the control voltage applied to the terminals 152 and the terminals 153 determines the threshold values OM2, OM3, and OM4. By increases or decreases in the control voltages applied to terminals 152 and 153, the threshold values OM2, OM2, and the threshold values OM3, OM4 may be correspondingly increased or decreased. However, the difference between the thresholds OM2, OMS or between the thresholds OM3, OM., is maintained constant and independent of the variation in the control voltage.

Gate elements of the characteristics herein described are per se well known in the art; a suitable circuit for such a purpose is shown in FIG. 3 presently to be described.

The output terminals of the gate elements 141, 142, 143 are connected to conventional frequency meters whose outputs are connected through leads 156, 157, 158 to galvanometer coils 159, 160, 161 respectively. The frequency meters in FIGS. l and 5 are designated by the letter F and may be of conventional construction. (See, for example, Type SC-34 precision ratemeter manufactured by Tracerlab, Inc.) The `galvanometer coils have attached thereto suitable mirrors in a manner well known to those skilled in the art and are adapted to reflect beams of light `derived from a source 162, thereby `effectively producing on the sensitive iilm 119 a record comprising three traces designated yas 163, 164, 165, respectively, and representing the variations of the voltage applied to the galvanometer coils 159, 160, 161, respectively, as shown in the tigure. A shaft 19t) is driven by a motor or a suitable clock mechanism 191 and carries a spool 118 for moving the photographic lm from a feed spool 120 to the take-up spool 118.

Thus the trace 163 represents the variation of the rate of incidence of photons or quanta within the energy range represented by pulses in Group I and the traces 164 and 165 represent the corresponding variations of the rate of incidence of photons or quanta within the energy ranges represented by the pulses in Groups II and III, respectively.

I shall now describe the operation of the stabilizing apparatus which comprises crystal 70 and gate elements 144 and 145.

Assume first that the photomultiplier is performing under normal operating conditions that are characterized by a certain known and predetermined sensitivity of the photomultiplier. Under these conditions the reference pulses of light emitted by the auxiliary crystal 70 and impinging upon the photocat-hode 23 causes an emission at the output terminals 71 of successive current impulses of uniform magnitude ON1, as shown in lFIG. 2. These impulses have been designated in FIG. 2 by numerals 3, 8, 12, and 18. These impulses cannot be transmitted through any of the gates 141, 142, 143. They are, moreover, too large to be transmitted through the gate 144 and too small to be transmitted through the gate 145.

Assume now that the sensitivity of the photomultiplier decreases. Consequently, the current pulses across the output terminals 71 that originate from crystal 71) decrease in size, and when they reach the magnitude ON2 they pass through the gate 144, producing a voltage across the output terminals of said gate and across the leads 170.

Assume instead that the sensitivity of the photomultiplier increases. This causes the reference impulses caused by crystal 70 to increase in size, and when they` reach the magnitude ON3 they pass through the gate 145, Iproducing a voltage across the leads 171.

Thus, when the sensitivity of the photomultiplier decreases a voltage appears across the terminals 170, and when the sensitivity increases, a voltage appears across the terminals 171. The terminals 170, 171 are respectively applied to excitation windings 172, 173 of a D.C. motor 174, said motor receiving its armature current supply from a battery 175. The windings 172, 173 are Wound and connected to produce two opposing magnetic fluxes, responsively to currents from terminals 171) and 171 respectively.

The motor 174 is adapted to displace angularly a rotatable conductive member 175e by means of a shaft 176. When the excitation winding is energized by the voltage output from the gate 144, the member turns in the clockwise direction, whereas current in winding 173 causes the member 175 to turn in the anti-clockwise direction.

One terminal 17'7 of the member 175 at the point o rotation is connected to a lead 173 and the other terminal 179 is slidingly engaged on a fixed semicircular resistor 180, said resistor having its two terminals 181, 182 connected to a battery 183.

The voltage between the grounded terminal 182 and lead 178 decreases when the member 175 rotates clockwise and increases when it rotates anti-clockwise. This voltage is simultaneously transmitted to the control terminals 151, 152, 153, 154, yand 155 of the gates 141, 142, 143, 144, and 145, respectively.

In order to understand the operation of the abovedescribed compensating system, assume that the sensitivity of the photomultiplier decreases. The impulses corresponding to photons of the Group I do not fall any longer within a range of magnitudes OM1, OM2 shown in FIG. 2. They fall within a lower range of magnitudes dened by limits OMl1 and CM2l which 7 are respectively below the corresponding limits CM1 and CM2 as shown in FIG. 2. Similarly, the impulses corresponding to gamma rays of Group Il and Group III do not fall any more within m-agnitude ranges CM2, OM3 and OMS, CM4, respectively, but within lower ranges of magnitudes defined by limits CM21, CM31, and OM31, OM41, respectively.

It is therefore apparent that when the sensitivity of the photomultiplier is decreased, the gates 141, 142, 143 are not adapted any more to transmit impulses that are caused by photons belonging to the energy Groups l, II, and III, respectively. It is therefore necessary to modify the transmitting characteristics of the gates 141, 142, and 143, so as to lower the bands of magnitudes from the positions M1, M2; M2, M3; and M3, M4 to the positions M11, M21; M21, M31; and M31, M41. This is effected by means of the control voltage appearing across the output terminals 170 of the `gate 144 in the manner hereinabove described. Said control voltage causes the rotation of the shaft 175 in a clockwise direction. It is apparent that as the shaft 175 rotates, the control voltages -applied to the terminals 151 to 155 decrease in magnitude and cause a progressive downward shift of the threshold values of the corresponding gates 141 to 145. In particular, the range of magnitude transmitted through the gate 144 is not any more defined by the magnitude CN2 but by a lower value. Consequently, the reference impulses caused by the auxiliary crystal 70 cannot pass any longer through the gate 144. Thus the voltage across the terminals 170 drops to zero and consequently the member 175 stops rotating and reaches a stationary position corresponding to a decrease in the control voltages to the terminals 151, 152, and 153 by a definite amount. This amount is such that the new thresholds 4corresponding to the gate 141 are not any more CM1, CM2 but CM11, OM21. The new thresholds corresponding to the gate 142 are not any more CM2, CM3 but CM21, CM3l and the new threshold values corresponding to the gate 143 are not any more CM3, CM., but OM31, OM41.

Thus, when the sensitivity of the photomultiplier decreases, the thresholds of the gates 141, 142, 143 adjust themselves automatically so that the gate 141 will accept all the impulses originated by gamma rays of the Groups II and lII, respectively. A similar automatic adjustment, out in the opposite direction, takes place when the sensitivity increases.

Consider now FIG. 3 showing a schematic diagram of a gate suitable for use as one of those designated by numerals 141-145 in FIG. l. The gate has input terminals 221, output terminals 235 and control terminals 236. The control terminals 236 may be any of those designated by 151-155 in FIG. l and the output leads may be any of those designated by 156a, 1570, 1580, 171m, 171a in FIG. 1.

The essential element of the gate consists of a pulseheight selector comprising two individual channels designated as A and B. The pulse-height selector channel is arranged to give across its output terminals a voltage pulse only when the input pulse applied to terminals 221 is contained within a predetermined range of magnitudes constituting the transmission band. This range of magnitudes is determined by the control voltage applied to the terminals 236. This is, with a certain value for the control voltage the circuit will pass only input voltage pulses within a predetermined band of magnitudes. If the input voltage pulses are outside the band no output will be produced.

Assume now that n impulses having magnitudes within the transmission band enter at the input terminals 221. These impulses produce across the terminals 252, 266 n voltage impulses, each of said voltage impulses having a very short duration. The channel A comprises input terminals 221, resistor 254, and triode 256, whose cathode 257 is connected to the positive terminal of biasing battery 258, its negative terminal being connected to the control terminals 236 and thence to ground. The plate 260 of this triode is connected to the output terminal 252 and to resistor 261 which in turn is connected to the positive terminal of battery 262. The negative side of battery 262 is grounded.

The channel B comprises input terminals 221, resistor 267, and triode 269, whose cathode 270 is connected to the positive terminal of biasing battery 271, its negative terminal being connected to the control terminals 236 and thence to ground. The plate 272 of the triode 269 is connected to the output terminal 266 and to resistor 274 which in turn is connected to the positive terminal of battery 262.

In reference now to channel A, it is apparent that we obtain at the output terminals 252, 253 only those input pulses that are capable of overcoming the biasing voltage of the tube 256. Assume that the voltage of the battery 258 is El and that the voltage applied to the control terminals 236 is Ec. Then the total biasing voltage for tube 256 is E14-Ec. Therefore, only the impulses that are capable of exceeding the threshold value provided by the total biasing voltage are transmitted through the channel A and appear across the output terminals 252, 253.

Similarly, in the channel B only those input pulses appear across the output terminals 266, 263 that are capable of overcoming the total biasing voltage of the tube 269. Assume that the Voltage of the battery 271 is E2. Then the total biasing voltage of the tube 269 is E24-Ec. Consequently, only those impulses that are capable of exceeding the threshold value determined by 1152+113c cause pulses to appear across the terminals 266, 253.

The two output voltages across the terminals 252, 253 and 266, 253 are in opposition, so that the resultant output between the terminals 252, 266 is equal to their difference. Consider now three cases designated as (a), (b), and (c).

Case (a): The impulse applied to the terminals 221 has a value below the threshold voltages of the tubes 256 and 269. Consequently, no plate currents will be delivered by these tubes and no voltage will appear across the terminals 252, 266.

Case (b): The impulse applied to the terminals 221 has a value above the threshold voltages of the tubes 256 and 269. Consequently, both tubes deliver plate currents, and two short voltage impulses appear substantially simultaneously across the output terminals 252, 253 and 266, 253. Since these two voltages are substantially equal, little or no resultant voltage appears across the terminals 252, 266.

Case (c): The pulse applied to the terminals 221 has a value smaller than the threshold of the tube 269, but larger than the threshold of the tube 256. Consequently, a plate current will pass through the tube 256 and no plate current will pass through the tube 269. Consequently, no voltage will be produced across the terminals 266, 253 and a short voltage impulse will appear across the terminals 252, 253. We obtain, therefore, across the terminals 266, 252 a voltage pulse. It is thus apparent that only those pulses that are comprised within the range determined by the value E14-Ec and E2-I-Ec produce corresponding output impulses across the terminals 266, 252.

The output impulses derived from terminals 266, 252 are applied through blocking capacitors 281 and 282 to an amplifier 291, the output of which is connected to leads 235 which represent the leads 171a or 170:1 or 158a or 157a or 156g in FIG. l. As shown in FIG. l, these leads are connected to the input terminals of frequency meters F. Again referring to FIG. l, consequently we obtain across the output terminals of the respective frequency meters F D.C. voltages representing the respective rates of occurrence of the pulses passed by the .respective pulse-height selectors.

If we refer now to the gate 141 of FIG. 1 at normal operating condition, then the value E24-Ec determines the lower threshold. If the sensitivity of the photomultiplier decreases then the control voltage applied to the terminal 141 decreases by an amount AEc and assumed a new value Ec-AEC. Then the upper threshold assumes a new value corresponding to Ez-l-Ec-AEC and the lower threshold assumes a new value corresponding to El-l-Ec-AEc It is apparent that the width of the transmitted band is determined by E2-E1 and is substantially independent of the Value of the control voltage. When the control voltage increases, the pass band is shifted upward; when the control voltage decreases, the pass band is shifted downward.

Similar relationships hold for all the remaining gates 142-145. It should be noted that under normal conditions the gates 143, 144, 145 admit very narrow bands comprising the magnitudes OC1, ODI, and CD2, respectively. Thus in case of the gate 143 the value ETI-Ec determines a value slightly over OC, the value El-l-Ec dete-rmines a value slightly below OC.

Other types of controllable gate circuits are well known in the art and may be used in my invention. The FIG. 3 circuit is described herein merely by way of example and for the sake of completeness.

The foregoing description of a typical embodiment of my invention is intended to be illustrative only. The scope of my invention is to be determined primarily by reference to the appended claims.

I claim:

1. The combination comprising a scintillation-type radiation detector including a photomultiplier, adapted to produce rst electric pulses in response to light flashes produced by incident nuclear radiation, a source of reference scintillations comprising a scintillating phosphor and a radioactive substance at least partially dispersed therewithin, said substance being characterised by emission of nuclear radiation of known characteristics, said source being optically coupled to said photomultiplier and operative to produce in the output thereof second electric pulses representing said known radiation, a system fed by said photomultiplier for measuring the respective magnitudes of said first pulses and said secon-d pulses, and means for comparing said magnitudes.

2. The combination comprising a scintillation-type radiation detector including a photomultiplier, adapted to produce first electric pulses in response to light flashes produced by incident nuclear radiation, a source `of reference scintillations comprising a Iscintillating phosphor and a radioactive substance at least partially dispersed therewithin, said substance being characterized by emission of nuclear radiation of known characteristics, said source being optically coupled to said photomultiplier and operative to produce in the output thereof second electric pulses representing said known radiation, a system fed by said photomultipliei for measuring the respective magnitudes of said first pulses and said second pulses, means for comparing said magnitudes, and means for adjusting the transmission characteristics of one of the elements of said combination responsively to variations. in the magnitude of said second pulses.

References Cited by the Examiner UNITED STATES PATENTS 2,913,669 11/1959 Hebert, Ir. 250--71.5 X

RALPH G. NILSON, Primary Examiner.

ARCHIE BORCHELT, Examiner. 

1. THE COMBINATION COMPRISING A SCINTILLATION-TYPE RADIATION DETECTOR INCLUDING A PHOTOMULTIPLIER, ADAPTED TO PRODUCE FIRST ELECTRIC PULSES IN RESPONSE TO LIGHT FLASHES PRODUCED BY INCIDENT NUCLEAR RADIATION, A SOURCE OF REFERENCE SCINTILLATIONS COMPRISING A SCINTILLATING PHOSPHOR AND A RADIOACTIVE SUBSTANCE AT LEAST PARTIALLY DISPERSED THEREWITHIN, SAID SUBSTANCE BEING CHARACTERISED BY EMISSION OF NUCLEAR RADIATION OF KNOWN CHARACTERISTICS, SAID SOURCE BEING OPTICALLY COUPLED TO SAID PHOTOMULTIPLIER AND OPERATIVE TO PRODUCE IN THE OUTPUT THEREOF SECOND ELECTRIC PULSES REPRESENTING SAID KNOWN RADIATION, A SYSTEM FED BY SAID PHOTOMULTIPLIER FOR MEASURING THE RESPECTIVE MAGNITUDES OF SAID FIRST PULSES AND SAID SECOND PULSES, AND MEANS FOR COMPARING SAID MAGNITUDE. 